Multilayer optical films are known. Such films typically incorporate a large number of very thin layers of different light transmissive materials, the layers being referred to as microlayers because they are thin enough so that the reflection and transmission characteristics of the optical film are determined in large part by constructive and destructive interference of light reflected from the layer interfaces. Depending on the amount of birefringence (if any) exhibited by the individual microlayers, and the relative refractive index differences for adjacent microlayers, and also on other design characteristics, the multilayer optical films can be made to have reflection and transmission properties that may be characterized as a reflective polarizer in some cases, and as a mirror in other cases, for example.
Reflective polarizers composed of a plurality of microlayers whose in-plane refractive indices are selected to provide a substantial refractive index mismatch between adjacent microlayers along an in-plane block axis and a substantial refractive index match between adjacent microlayers along an in-plane pass axis, with a sufficient number of layers to ensure high reflectivity for normally incident light polarized along one principal direction, referred to as the block axis, while maintaining low reflectivity and high transmission for normally incident light polarized along an orthogonal principal direction, referred to as the pass axis, have been known for some time. See, e.g., U.S. Pat. No. 3,610,729 (Rogers), U.S. Pat. No. 4,446,305 (Rogers et al.), and U.S. Pat. No. 5,486,949 (Schrenk et al.).
More recently, researchers from 3M Company have pointed out the significance of layer-to-layer refractive index characteristics of such films along the direction perpendicular to the film, i.e. the z-axis, and shown how these characteristics play an important role in the reflectivity and transmission of the films at oblique angles of incidence. See, e.g., U.S. Pat. No. 5,882,774 (Jonza et al.). Jonza et al. teach, among other things, how a z-axis mismatch in refractive index between adjacent microlayers, more briefly termed the z-index mismatch or Δnz, can be tailored to allow the construction of multilayer stacks for which the Brewster angle—the angle at which reflectance of p-polarized light at an interface goes to zero—is very large or is nonexistent. This in turn allows for the construction of multilayer mirrors and polarizers whose interfacial reflectivity for p-polarized light decreases slowly with increasing angle of incidence, or is independent of angle of incidence, or increases with angle of incidence away from the normal direction. As a result, multilayer films having high reflectivity for both s- and p-polarized light for any incident direction in the case of mirrors, and for the selected direction in the case of polarizers, over a wide bandwidth, can be achieved.
Some multilayer optical films are designed for narrow band operation, i.e., over a narrow range of wavelengths, while others are designed for use over a broad wavelength range such as substantially the entire visible or photopic spectrum, or the visible or photopic wavelength range together with near infrared wavelengths, for example. Over the years, designers and manufacturers of the latter type of films, i.e., broadband multilayer optical films, have had to deal with the issue of color. The color issue often arises when the film is intended for use in a visual display system, e.g., where the film is a broadband reflective polarizer or a broadband mirror, and the display system is a liquid crystal display, luminaire, or backlight. In such systems, it is typically undesirable for the film to impart a significant colored (non-white) appearance to the display, whether at normal incidence or for obliquely incident light. The colored appearance occurs when the film has transmission or reflection characteristics that are not uniform over the visible portion of the spectrum. In the case of coextruded polymeric multilayer optical films, such non-uniformities are typically the result of imperfect control of the layer thickness profile of the film relative to a target profile. To avoid the color issue, polymeric multilayer optical films are often designed to provide along their principal axes either extremely low reflectivity and high transmission (e.g. for a pass axis of a reflective polarizer) or extremely high reflectivity and low transmission (e.g. for a block axis of a reflective polarizer, or for any in-plane axis of a reflective mirror film). Forcing the reflectivity to extremely low or extremely high values (and the transmission to extremely high or extremely low values, respectively) results in low-color broadband films, because the magnitude of spectral non-uniformities in reflection or transmission is smaller for a given variability in layer thickness profile when the nominal reflectivity is near 0 (0%) or 1 (100%), and the nominal transmission is near 1 or 0, respectively.
Recently, broadband polymeric multilayer optical films have been proposed that have intermediate amounts of reflectivity and transmission for light polarized parallel to at least one principal optic axis so that some significant amount of incident light is reflected, and another significant amount of the incident light (typically, the remainder of the incident light that is not reflected) is transmitted. Such films are referred to herein as partially reflecting multilayer optical films, or partially transmitting multilayer optical films. One approach to addressing color issues in such films is to provide them with only a single packet of microlayers with a carefully tailored layer thickness profile, and to manufacture them without the use of any layer multiplier devices, to provide maximum control of the layer thickness profile and a corresponding minimum spectral variability in transmission or reflection over the visible wavelength range. Reference is made, for example, to PCT publication WO 2009/0123928 (Derks et al.), “Low Layer Count Reflective Polarizer With Optimized Gain”.